Gas turbine engine combustion chamber

ABSTRACT

A combustion chamber which has a primary combustion zone and a secondary combustion zone is provided with a plurality of secondary fuel and air mixing ducts arranged around the primary combustion zone. The secondary fuel and air mixing ducts are defined by a pair of annular walls and by a plurality of walls extending radially between the annular walls. Each secondary fuel and air mixing duct has an aperture to direct a fuel and air mixture into the secondary combustion zone. The apertures have the same flow area. Each secondary fuel and air mixing duct has one or more fuel injectors to inject fuel into the upstream end of the secondary fuel and air mixing duct. This arrangement ensures that the fuel/air ratio emitted from each aperture is within 3.0% of the mean fuel/air ratio of all the apertures even though the air flow to the secondary fuel and air mixing ducts is non-uniform

This application claims benefit of international application PCT/GB94/01135 filed May 24, 1994.

FIELD OF THE INVENTION

The present invention relates to a gas turbine engine combustionchamber.

BACKGROUND OF THE INVENTION

In order to meet the emission level requirements, for industrial lowemission gas turbine engines, staged combustion is required in order tominimise the quantity of the oxides of nitrogen (NOx) produced.Currently the emission level requirement is for less than 25 volumetricparts per million of NOx for an industrial gas turbine exhaust. Thefundamental way to reduce emissions of nitrogen oxides is to reduce thecombustion reaction temperature, and this requires premixing of the fueland all the combustion air before combustion takes place. The oxides ofnitrogen (NOx) are commonly reduced by a method which uses two stages offuel injection. Our UK patent no. 1489339 discloses two stages of fuelinjection to reduce NOx. Our International patent application no.WO92/07221 discloses two and three stages of fuel injection. In stagedcombustion, all the stages of combustion seek to provide lean combustionand hence the low combustion temperatures required to minimise NOx. Theterm lean combustion means combustion of fuel in air where the fuel toair ratio is low i.e. less than the stoichiometric ratio. In order toachieve the required low emissions of NOx and CO it is essential to mixthe fuel and air uniformly so that it has less than a 3.0% variationfrom the mean concentration before the combustion takes place.

The industrial gas turbine engine disclosed in our International patentapplication no. WO92/07221 uses a plurality of tubular combustionchambers, whose longitudinal axes are arranged in generally radialdirections. The inlets of the tubular combustion chambers are at theirradially outer ends, and transition ducts connect the outlets of thetubular combustion chambers with a row of nozzle guide vanes todischarge the hot exhaust gases axially into the turbine sections of thegas turbine engine. Each of the tubular combustion chambers has anannular secondary fuel and air mixing duct which surrounds the primarycombustion zone. A plurality of equi-spaced secondary fuel injectors arearranged to inject fuel into the upstream end of the annular secondaryfuel and air mixing duct. The annular secondary fuel and air mixing ducthas a plurality of equi-spaced outlet apertures to direct the fuel andair mixture into the secondary combustion zone. Each of the tubularcombustion chambers of the three stage variant also has an annulartertiary fuel and air mixing duct which surrounds the secondarycombustion zone. A plurality of equi-spaced tertiary fuel injectors arearranged to inject fuel into the upstream end of the annular tertiaryfuel and air mixing duct. The annular tertiary fuel and air mixing ducthas a plurality of outlet apertures to direct the fuel and air mixtureinto the tertiary fuel and air mixing zone.

Unfortunately the flow of air into the tubular combustion chambers isnot uniform, this is because of an asymmetric flow of air from adiffuser at the downstream end of the gas turbine engine compressor tothe tubular combustion chambers. Each of the secondary fuel injectorspasses identical fuel flows and therefore a non uniform fuel and airmixture is created at the points of injection due to the non uniform airflow. The fuel and air mixture directed from the outlet apertures intothe secondary combustion zone is non uniform. Similarly the fuel and airmixture directed from the outlet apertures of the tertiary mixing ductinto the tertiary combustion zone will be non uniform. This increasesthe emissions of NOx to above the acceptable levels.

An initial solution for the problem was to redistribute the fuel tomatch the air mass flow distribution by adjusting the fuel hole sizes ofthe individual fuel injectors. This requires all of the fuel injectorsto be unique in fuel hole diameters and position of the fuel holes tomatch the air mass flow to achieve the required uniformity of mixing.The air mass flow distribution also varies with the operating powerrange of the engine. However redistributing the fuel to match the airmass flow distribution would not achieve the required 3.0% variation inconcentration uniformity at all powers and hence emissions of NOx wouldbe above the acceptable levels.

Another solution for the problem was to fit air guidance devicesupstream of the secondary fuel and air mixing duct, and tertiary fueland air mixing duct, to create a uniform air mass flow at the intakes ofthe secondary fuel and air mixing duct, and tertiary fuel and air mixingduct. Unfortunately any minor changes in the air guidance devices formedduring the production processes result in relatively large changes inair mass flow distribution i.e. greater than the 3.0% variation inconcentration uniformity.

A further solution for the problem was to redistribute the air mass flowupstream of the intakes of the secondary fuel and air mixing duct, andtertiary fuel and air mixing duct, using a flow distributor which usesits pressure drop to create uniform flow through each of its flowroutes. Unfortunately an increase in system pressure drop is notacceptable because this reduces the surge margin of the compressor andalso reduces the thermal efficiency of the engine i.e. increases theengine fuel consumption.

The only acceptable solution therefore must be tolerant to upstream airflow variations without increasing the system pressure loss.

EPO388886A discloses a combustor for burning of fuel by premixing fuelwith air in a number of premix flame forming nozzles which inject thepremixed fuel and air into a secondary combustion zone. Fuel injectorsare provided to inject fuel into the premix flame forming nozzlesdownstream of the intakes of the premix flame forming nozzles.

The present invention seeks to provide a novel gas turbine enginecombustion chamber which overcomes the above mentioned problem.

Accordingly the present invention provides a gas a turbine enginecombustion chamber comprising a primary combustion zone defined by atleast one peripheral wall and an upstream end wall connected to theupstream end of the at least one peripheral wall, the upstream end wallhas at least one aperture, primary air intake means and primary fuelinjector means to supply air and fuel respectively through the at leastone aperture into the primary combustion zone, a secondary combustionzone in the interior of the combustion chamber downstream of the primarycombustion zone, means to define a plurality of secondary fuel and airmixing ducts, each secondary fuel and air mixing duct has an outlet atits downstream end for discharging the fuel and air mixture into thesecondary combustion zone, each secondary fuel and air mixing duct hassecondary air intake means at its upstream end to supply air into thesecondary fuel and air mixing duct, each secondary fuel and air mixingduct has secondary fuel injector means arranged to supply fuel into thesecondary fuel and air mixing duct, each secondary fuel injector meansis located downstream of the secondary air intake means of theassociated secondary fuel and air mixing duct, the outlets of thesecondary fuel and air mixing ducts have substantially equal flow areasto produce substantially the same air flow rate through each of thesecondary fuel and air mixing ducts, the secondary fuel injector meansof each secondary fuel and air mixing duct is arranged to supplysubstantially the same flow rate of fuel so that the fuel to air ratioof the mixture leaving each of the secondary fuel and air mixing ductsis substantially the same.

Preferably the secondary fuel and air mixing ducts radially inwardly ofthe primary combustion zone, the secondary fuel and air mixing ducts aredefined at their radially inner extremity and radially outer extremityby a second pair of walls and a plurality of walls extending radiallybetween the second pair of annular walls.

Preferably at least one of the secondary fuel injector means comprises ahollow cylindrical member, the hollow cylindrical member has a pluralityof apertures spaced apart axially along the cylindrical member to injectfuel into the secondary fuel and air mixing duct.

The hollow cylindrical member may extend axially with respect to theaxis of the combustion chamber. The hollow cylindrical member may extendradially with respect to the axis of the combustion chamber. Theapertures in the hollow cylindrical member may be arranged to direct thefuel circumferentially.

Preferably the walls extending radially between the annular walls aresecured to both the annular walls.

Preferably the secondary fuel injector means for at least one of thesecondary fuel and air mixing ducts comprises two secondary fuelinjectors. The two secondary fuel injectors may be spaced apartcircumferentially relative to the axis of the combustion chamber.Preferably each secondary fuel injector is arranged to supply fuel tothe upstream end of the associated secondary fuel and air mixing duct.

Preferably the combustion chamber includes means to define a pluralityof tertiary fuel and air mixing ducts, each tertiary fuel and air mixingduct is in fluid communication at its downstream end with a tertiarycombustion zone in the interior of the combustion chamber downstream ofthe secondary combustion zone, each tertiary fuel and air mixing ducthas tertiary air intake means at its upstream end to supply air into thetertiary fuel and air mixing duct, each tertiary fuel and air mixingduct has tertiary fuel injector means arranged to inject fuel into thetertiary fuel and air mixing duct, the tertiary fuel and air mixingducts are arranged in an annulus outside the peripheral wall, eachtertiary fuel injector means is located downstream of the tertiary airintake means of the associated tertiary fuel and air mixing duct, eachtertiary fuel and air mixing duct has an outlet at its downstream endfor discharging the fuel and air mixture into the tertiary combustionzone, the outlets of the tertiary fuel and air mixing ducts havesubstantially equal flow areas to produce substantially the same airflow rate through each of the tertiary fuel and air mixing ducts, thetertiary fuel injector means of each tertiary fuel and air mixing ductis arranged to supply substantially the same flow rate of fuel so thatthe fuel to air ratio of the mixture leaving each of the tertiary fueland air mixing ducts is substantially the same.

Preferably the tertiary fuel and air mixing ducts are defined by aradially inner annular wall, a radially outer annular wall and aplurality of walls extending radially between the pair of annular walls,the radially extending walls are secured to at least one of the pair ofannular walls.

Preferably the tertiary fuel and air mixing ducts are arranged aroundthe combustion chamber.

The combustion chamber may be tubular, the peripheral wall of theprimary combustion zone is annular and the upstream end wall has asingle aperture, the plurality of tertiary fuel and air mixing ducts arearranged circumferentially in an annulus radially outwardly of thesecondary combustion zone.

Preferably at least one of the tertiary fuel injector means comprises ahollow cylindrical member, the hollow cylindrical member has a pluralityof apertures spaced apart axially along the cylindrical member to injectfuel into the tertiary fuel and air mixing duct.

The hollow cylindrical member may extend axially with respect to theaxis of the combustion chamber. The hollow cylindrical member may extendradially with respect to the axis of the combustion chamber. Theapertures in the hollow cylindrical member may be arranged to direct thefuel circumferentially.

Preferably the tertiary fuel injector means for at least one of thetertiary fuel and air mixing ducts comprises two tertiary fuelinjectors. The two tertiary fuel injectors may be spaced apart axiallyrelative to the axis of the combustion chamber. The two tertiary fuelinjectors may be spaced apart circumferentially relative to the axis ofthe combustion chamber.

The present invention also provides a gas turbine engine combustionchamber comprising a primary combustion zone defined by at least oneperipheral wall and an upstream end wall connected to the upstream endof the at least one peripheral wall, the upstream end wall has at leastone aperture, primary air intake means and primary fuel injector meansto supply air and fuel respectively through the at least one apertureinto the primary combustion zone, a secondary combustion zone defined bya downstream portion of the at least one peripheral wall, the secondarycombustion zone is in the interior of the combustion chamber downstreamof the primary combustion zone, secondary air intake means and secondaryfuel injector means to supply air and fuel respectively into thesecondary combustion zone, means to define a plurality of tertiary fueland air mixing ducts, each tertiary fuel and air mixing duct is in fluidflow communication at its downstream end with a tertiary combustion zonein the interior of the combustion chamber downstream of the secondarycombustion zone, each tertiary fuel and air mixing duct has tertiary airintake means at its upstream end to supply air into the tertiary fueland air mixing duct, each tertiary fuel and air mixing duct has tertiaryfuel injector means arranged to supply fuel into the tertiary fuel andair mixing duct, each tertiary fuel injector means is located downstreamof the tertiary air intake means of the associated tertiary fuel and airmixing duct, each tertiary fuel and air mixing duct has an outlet at itsdownstream end for discharging the fuel and air mixture into thetertiary combustion zone, the outlets of the tertiary fuel and airmixing ducts have substantially equal flow areas to producesubstantially the same air flow rate through each of the tertiary fueland air mixing ducts, the tertiary fuel injector means of each fuel andair mixing duct is arranged to supply substantially the same flow rateof fuel so that the fuel to air ratio of the mixture leaving each of thetertiary fuel and air mixing ducts is substantially the same.

Preferably the tertiary fuel and air mixing ducts are arranged aroundthe combustion chamber.

Preferably the tertiary fuel and air mixing ducts are arranged in anannulus outside the peripheral wall, the tertiary fuel and air mixingducts are defined by a radially inner annular wall, a radially outerannular wall and a plurality of walls extending radially between thepair of annular walls, the radially extending walls are secured to atleast one of the pair of annular walls.

The present invention will be more fully described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 is a view of a gas turbine engine having a combustion chamberassembly according to the present invention.

FIG. 2 is an enlarged longitudinal cross-sectional view through thecombustion chamber shown in FIG. 1.

FIG. 3 is a further enlarged longitudinal cross-sectional view throughthe upstream end of the combustion chamber assembly shown in FIG. 2.

FIG. 4 is a cross-sectional view in the direction of arrows A--A in FIG.3.

FIG. 5 is a cross-sectional perspective view of the combustion chamberassembly shown in FIG. 2.

FIG. 6 is an enlarged longitudinal cross-sectional view through analternative combustion chamber assembly according to the presentinvention.

FIG. 7 is an enlarged longitudinal cross-sectional view through afurther alternative combustion chamber assembly according to the presentinvention.

FIG. 8 is an alternative longitudinal cross-sectional view through theupstream end of the combustion chamber assembly shown in FIG. 2.

An industrial gas turbine engine 10, shown in FIG. 1, comprises in axialflow series an inlet 12, a compressor section 14, a combustion chamberassembly 16, a turbine section 18, a power turbine section 20 and anexhaust 22. The turbine section 18 is arranged to drive the compressorsection 14 via one or more shafts (not shown). The power turbine section20 is arranged to drive an electrical generator 26 via a shaft 24.However, the power turbine section 20 may be arranged to provide drivefor other purposes. The operation of the gas turbine 10 is quiteconventional, and will not be discussed further.

The Combustion chamber assembly 16 is shown more clearly in FIGS. 2 to5. A plurality of compressor outlet guide vanes 28 are provided at theaxially downstream end of the compressor section 14, to which is securedat their radially inner ends an inner annular wall 30 which defines theinner surface of an annular chamber 32. A first passage 38 of a splitdiffuser is defined between an annular wall 34 and the upstream end ofthe inner annular wall 30 and a second passage 40 of the split diffuseris defined between the annular wall 34 and a further annular wall 36.The downstream end of the inner annular wall 30 is secured to theradially inner ends of a row of nozzle guide vanes 42 which direct hotgases from the combustion chamber assembly 16 into the turbine section18.

The combustion chamber assembly 16 comprises a plurality of, for examplenine, equally circumferentially spaced tubular combustion chambers 44.The axes of the tubular combustion chambers 44 are arranged to extend ingenerally radial directions. The inlets of the tubular combustionchambers 44 are at their radially outermost ends and their outlets areat their radially innermost ends.

Each of the tubular combustion chambers 44 comprises an upstream wall 46secured to the upstream end of an annular wall 48. A first, upstream,portion 50 of the annular wall 48 defines a primary combustion zone 52,and a second, downstream, portion 54 of the annular wall 48 defines asecondary combustion zone 56. The second portion 54 of the annular wall48 has a greater diameter than the first portion 50. The downstream endof the first portion 50 has a frustoconical portion 58 which reduces indiameter to a throat 60. A third frustoconical portion 62 interconnectsthe throat 60 at the downstream end of the first portion 50 and theupstream end of the second portion 54.

A plurality of equally circumferentially spaced transition ducts 64 areprovided, and each of the transition ducts 64 has a circularcross-section at its upstream end. The upstream end of each of thetransition ducts 64 is located coaxially with the downstream end of acorresponding one of the tubular combustion chambers 44, and each of thetransition ducts 64 connects and seals with an angular section of thenozzle guide vanes 42.

A plurality of cylindrical casings 66 are provided, and each cylindricalcasing 66 is located coaxially around a respective one of the tubularcombustion chambers 44. Each cylindrical casing 66 is secured to arespective boss 68 on an annular engine casing 70. A number of chambers72 are formed between each tubular combustion chamber 44 and itsrespective cylindrical casing 66.

The upstream end of each transition duct 64 and the downstream end of acorresponding tubular combustion chamber 44 are located in a respectiveannular mounting structure 74 which is secured to one of the bosses 68by one of the cylindrical casings 66. The annular mounting structure 74is provided with apertures 76 to allow the flow of air from chamber 32into the chambers 72.

The upstream wall 46 of each of the tubular combustion chambers 44 hasan aperture 78 to allow the supply of air and fuel into the primarycombustion zone 52. A first radial flow swirler 80 is arranged coaxiallywith the aperture 78 in the upstream wall 46 and a second radial flowswirler 82 is arranged coaxially with the aperture 78 in the upstreamwall 46. The first radial flow swirler 80 is positioned axiallydownstream, with respect to the axis of the tubular combustion chamber,of the second radial flow swirler 82. The first radial flow swirler 80has a plurality of fuel injectors 84, each of which is positioned in apassage formed between two vanes of the swirler. The second radial flowswirler 82 has a plurality of fuel injectors 86, each of which ispositioned in a passage formed between two vanes of the swirler. Thefirst and second radial flow swirlers 80 and 82 are arranged such theyswirl the air in opposite directions. For a more detailed description ofthe use of the two radial flow swirlers and the fuel injectorspositioned in the passages formed between the swirl vanes see ourInternational Patent Application No WO92/07221. The primary fuel and airis mixed together in the passages between the vanes of the first andsecond radial flow swirlers 80 and 82.

A plurality of secondary fuel and air mixing ducts 88 are provided foreach of the tubular combustion chambers 44. The secondary fuel and airmixing ducts 88 are arranged circumferentially in an annulus around theprimary combustion zone 52. Each of the secondary fuel and air mixingducts is defined between a second annular wall 90, a third annular wall92 and by walls 94 which extend radially between the second and thirdannular walls 90 and 92. The second annular wall 90 defines the radiallyouter extremity of each of the secondary fuel and air mixing ducts 88and the third annular wall 92 defines the radially inner extremity ofeach of the secondary fuel and air mixing ducts 88. The walls 94separate the individual secondary fuel and air mixing ducts 88. Theaxially upstream end 96 of the third annular wall 92 is curved radiallyoutwardly so that it is spaced axially from the upstream end of thesecond annular wall 90. The upstream end of the third annular wall 92 issecured to a side plate of the first radial flow swirler 80. Each of thesecondary fuel and air mixing ducts 88 has a secondary air intake 98defined axially between the upstream end of the second annular wall 90,the upstream end of the third annular wall 92 and the upstream ends ofthe walls 94 which also extend axially between the second and thirdannular walls 90 and 92 respectively at this position. For examplesixteen secondary fuel and air mixing ducts 88 are provided.

A plurality of secondary fuel injectors 100 are provided, at least onesecondary fuel injector 100 is provided per secondary fuel and airmixing duct 88. Each of the secondary fuel and air injectors 100comprises a hollow cylindrical member which extends axially with respectto the tubular combustion chamber 44. Each of the hollow cylindricalmembers 100 passes through the upstream end of the third annular wall 92to supply fuel into the upstream end of the secondary fuel and airmixing duct 88. The hollow cylindrical member is provided with aplurality of apertures 102 through which the fuel is injected into thesecondary fuel and air mixing duct 88. The apertures 102 are of equaldiameters and are spaced apart axially along the hollow cylindricalmember at suitable positions, and the apertures 102 in the hollowcylindrical member are arranged at diametrically opposite sides of thehollow cylindrical member so that the fuel injectors 100 are arranged toinject the fuel circumferentially with respect to the axis of thetubular combustion chamber 44. In this example two fuel injectors 100are provided for each secondary fuel and air mixing duct 88. Thesecondary fuel injectors are spaced apart circumferentially with respectto the axis of the tubular combustion chamber 44.

Each second and third annular wall 90 and 92 is arranged coaxiallyaround the first portion 50 of the annular wall 48. At the downstreamend of each secondary fuel and air mixing duct 88, the second and thirdannular walls 90 and 92 are secured to the respective thirdfrustoconical portion 62, and each frustoconical portion 62 is providedwith a plurality of equi-circumferentially spaced apertures 104 whichare arranged to direct fuel and air into the secondary combustion zone56 in the tubular combustion chamber 44, in a downstream directiontowards the axis of the tubular combustion chamber 44. The apertures 104may be circular or slots. Each of the apertures 104 is arranged to allowthe fuel and air mixture from one of the secondary fuel and air mixingducts 88 to flow into the secondary combustion zone 56. The apertures104 are of equal flow area.

The operation of the gas turbine combustion chamber is substantially asdescribed in our International Patent Application No WO92/07221 and thisshould be consulted for a more complete description.

The use of a single annular secondary fuel and air mixing duct in ourInternational Patent Application No WO92/07221 results in an air andfuel mixture which has a variation in concentration of more than 3.0%from the mean concentration and this results in NOx levels greater than25 volume parts per million (vppm).

The use of a plurality of secondary fuel and air mixing ducts each ofwhich has an aperture into the secondary combustion zone enables the airand fuel mixture to have a variation in concentration less than the 3.0%from the mean concentration and hence results in NOx less than 25 vppm.

The mass flow rate through each secondary fuel and air mixing duct 88 isdominated by the aperture 104 exit area and the pressure drop across it.The exit areas of the apertures 104 are controlled to be within 1.0%more, or less of the required flow area and the upstreamvelocity/pressure variations are negligible compared to the pressureacross the exit area of the aperture 104. This results in the air massflow entering each secondary fuel and air mixing duct 88 being within1.0% more, or less, of the mean mass flow through all of the fuel andair mixing ducts 88. Each duct 88 is supplied by two secondary fuelinjectors 100, each of which is within 2.0% of the mean area, theoverall resultant concentration is within 3.0% of the meanconcentration. This arrangement ensures that the fuel/air ratio emittedfrom each aperture 104 is within 3.0% of the mean fuel/air ratio of allthe apertures 104. The arrangement has been tested and has produced NOxand CO exhaust emissions of less that 10 vppm throughout its fulloperating power range, ie at temperatures in the secondary combustionzone of 1600° K. to 1750° K.

A feature of the invention is that the adjacent mixing ducts share acommon wall. The walls 94 separating the individual secondary fuel andair mixing ducts 88 extend from the secondary air intake 98 at theirupstream ends all the way to the frustoconical portion 62 and the walls94 are secured to the frustoconical portion 62. Also the walls 94 extendradially between and are secured to both the annular walls 90 and 92.Thus the secondary fuel and air mixing ducts 88 are completely separatedmechanically by the walls 94.

The use of the secondary annular mixing duct which is subdivided byradially extending walls 94 creates uniform fuel and air mixtures,independent of upstream air maldistributions. The fuel and air mixtureis injected as discrete jets into the secondary combustion zone 52. Thesecondary annular mixing duct subdivided by the radially extending walls94 creates the minimum amount of blockage and flow disturbance to theairflow around the combustion chamber. This is of particular importanceto the tubular combustion chambers whose axis are arranged in generallyradial directions, because the air flow has to turn through 180°. Thisarrangement of the secondary fuel and air mixing ducts 88 has a minimumdiameter increase greater than the primary combustion zone 52, to createthe maximum annular flow area between the outer annular wall 90 of thesecondary fuel and air mixing duct 88 and the cylindrical casing 66 inthe chambers 72. The air flow to the secondary fuel and air mixing ducts88 in the chamber 72 is counter to the flow in the secondary fuel andair mixing ducts 88, and the air flow in the chamber 72 is at a lowvelocity to create a high flow acceleration into the secondary fuel andair mixing ducts 88 in order to prevent flow separation as the air flowturns through 180°.

The invention has been described with reference to staged combustion intubular combustion chambers, it may also be applied to staged combustionin annular combustion chambers as shown in FIG. 6. An annular combustionchamber 110 has an annular primary combustion zone 52 and an annularsecondary combustion zone 56 defined between a radially outer annularwall 46 and a radially inner annular wall 146. A plurality of secondaryfuel and air mixing ducts 88 are arranged in a first annulus radiallyoutwardly of the annular primary combustion zone 52 and a plurality ofsecondary fuel and air mixing ducts 88 arranged in a second annulusradially inwardly of the annular primary combustion zone 52. Thesecondary fuel and air mixing ducts 88 are defined between two annularwalls 90 and 92 and by walls 94 extending radially between the walls 90and 92. A fuel injector 100 is positioned at the upstream end of eachsecondary fuel and air mixing duct 88, and extends radially with respectto the axis of the combustion chamber 110. The secondary fuel and airmixing ducts 188 are defined between two annular walls 190 and 192 andby walls 194 extending radially between the walls 190 and 192. A fuelinjector 200 is positioned at the upstream end of each secondary fueland air mixing duct 188, and extends radially with respect to the axisof the combustion chamber 110. Each of the secondary fuel and air mixingducts 88 communicates via a respective aperture 104 in the annular wall46 to allow the fuel and air mixture to flow into the secondarycombustion zone 56. The apertures 104 are of equal flow area. Each ofthe secondary fuel and air mixing ducts 188 communicates via arespective aperture 204 in the annular wall 146 to allow the fuel andair mixture to flow into the secondary combustion zone 56. The apertures204 are of equal flow area.

The invention is also applicable to the tertiary stage of three stagecombustion chamber as shown in FIG. 7. A tubular combustion chamber 210has a plurality of tertiary fuel and air mixing ducts 288 arranged in anannulus radially outwardly of a tertiary combustion zone 290. Thetertiary fuel and air mixing ducts 288 are defined between two annularwalls 290 and 292 and by walls 294 extending radially between the walls290 and 292. A fuel injector 300 is positioned at the upstream end ofeach tertiary fuel and air mixing duct 288, and extends axially withrespect to the axis of the combustion chamber 210. Each of the tertiaryfuel and air mixing ducts 288 communicates via a respective aperture 304in the annular wall 46 to allow the fuel and air mixture to flow intothe tertiary combustion zone 290. The apertures 304 are of equal flowarea.

The invention has been described with reference to tubular and annularcombustion chambers, but the invention is applicable to combustionchambers of other shapes. The secondary fuel and air mixing ducts neednot be positioned around the primary combustion zone and the tertiaryfuel and air mixing ducts need not be positioned around the secondarycombustion zone.

In a further embodiment, shown in FIG. 8, the walls 94 of the secondaryfuel and air mixing ducts 88 do not extend the full distance to thefrustoconical portion 62. Deflecting member 95 are secured to theannular walls 90 and 92 to direct the fuel and air mixture at theappropriate angle through the apertures 104 into the secondarycombustion zone 56. The walls 94 extend a sufficient distance from theintakes 98 towards the members 95 to aerodynamically separate theairflows, such that there are no, or insignificant, mass flows betweenadjacent secondary fuel and air mixing ducts 88, ie the walls 94 mustextend a sufficient distance to control the flow of air. Similarly thewalls 94 do not extend the full radial distance between the annularwalls 90 and 92. The walls 94 extend a sufficient distance from one ofthe annular walls 90 or 92 respectively towards the other annular wall92 or 90 respectively to aerodynamically separate the airflows, suchthat there are no, or insignificant, mass flows between adjacentsecondary fuel and air mixing ducts 88. FIG. 8 shows one wall 94Asecured to the annular wall 90 and one wall 94B secured to the otherannular wall 92. The mass flow rate through the secondary fuel and airmixing ducts 88 is such that the air and fuel cannot turn through thegaps between the walls 94 and annular walls 90 and 92 or deflectingmembers 95.

Also the fuel injectors 100 in FIG. 8 are located at a position spacedfrom the intake 98. The fuel injectors 100 may be located at anyposition along the secondary air and fuel mixing ducts 88 which producesacceptable mixing of the fuel and air. The fuel injectors 100 must bedownstream of the intakes 98, and there must be a sufficient distancebetween the fuel injectors 100 and the apertures 104 to give therequired mixing. The fuel injectors 100 must be downstream of theintakes 100 so that the fuel is supplied into the airflow after it hasbeen divided into the individual secondary fuel and air mixing ducts 88in order to obtain the required fuel to air ratio at the aperture 104 ofeach duct.

Thus it can be seen that the invention provides a number of secondaryfuel and air mixing ducts for premixing the fuel and air before it issupplied into the secondary combustion zone. The main feature of thesepremixing ducts is that their outlets into the secondary combustion zoneare of substantially the same flow area, and thus each secondary fueland air premixing duct has substantially the same flow rate of airtherethrough. Furthermore the fuel injectors for each of the secondaryfuel and air mixing ducts are arranged to supply substantially the sameflow rate of fuel. Thus the fuel to air ratio of the mixture leavingeach of the secondary fuel and air mixing ducts is substantially thesame. Similarly each of the tertiary fuel and air mixing ducts havesubstantially the same outlet flow area, substantially the same air flowrate, and substantially the same flow rate of fuel supplied to it.

The invention also provides that the outlets of the secondary fuel andair mixing ducts may have different flow areas and thus different airflow rates. In this case the secondary fuel injectors have their fuelflow rates adjusted so that the fuel to air ratio of the mixture leavingeach of the secondary fuel and air mixing ducts is substantially thesame.

We claim:
 1. A gas turbine engine combustion chamber (44) comprising aprimary combustion zone (52) defined by at least one peripheral wall(48) and an upstream end wall (46) connected to the upstream end of theat least one peripheral wall (48), the upstream end wall (46) has atleast one aperture (78), primary air intake means (80,82) and primaryfuel injector means (84,86) to supply air and fuel respectively throughthe at least one aperture (78) into the primary combustion zone (52), asecondary combustion zone (56) in the interior of the combustion chamber(44) downstream of the primary combustion zone (52), means (90,92,94) todefine a plurality of secondary fuel and air mixing ducts (88), eachsecondary fuel and air mixing duct (88) has secondary air intake means(98) at its upstream end (96) to supply air into the secondary fuel andair mixing duct (88), each secondary fuel and air mixing duct (88) hassecondary fuel injector means (100) arranged to supply fuel into thesecondary fuel and air mixing duct (88), each secondary fuel injectormeans (100) is located downstream of the secondary air intake means (98)of the associated secondary fuel and air mixing duct (88), eachsecondary fuel and air mixing duct (88) has an outlet (104) at itsdownstream end for discharging the fuel and air mixture into thesecondary combustion zone (56), characterised in that the outlets (104)of the secondary fuel and air mixing ducts (88) have substantially equalflow areas to produce substantially the same air flow rate through eachof the secondary fuel and air mixing ducts (88), the secondary fuelinjector means (100) of each secondary fuel and air mixing duct (88) isarranged to supply substantially the same flow rate of fuel so that thefuel to air ratio of the mixture leaving each of the secondary fuel andair mixing ducts (88) is substantially the same.
 2. A combustion chamberas claimed in claim 1 in which the secondary fuel and air mixing ducts(88) are arranged in an annulus outside the peripheral wall (48), thesecondary fuel and air mixing ducts (88) are defined by a radially innerannular wall (92), a radially outer annular wall (90) and a plurality ofwalls (94) extending radially between the pair of annular walls (90,92),the radially extending walls (94) are secured to at least one of thepair of annular walls (90,92).
 3. A combustion chamber as claimed inclaim 2 in which the secondary fuel and air mixing ducts (88) arearranged around the combustion chamber (44).
 4. A combustion chamber asclaimed in claim 2 in which the combustion chamber is tubular, theperipheral wall (48) of the primary combustion zone (52) is annular andthe upstream end wall (46) has a single aperture (78), the secondaryfuel and air mixing ducts (88) are arranged around the primarycombustion zone (52), said plurality of secondary fuel and air mixingducts (88) being arranged circumferentially in an annulus radiallyoutwardly of the annular wall (48) of the primary combustion zone (52).5. A combustion chamber as claimed in claim 2 in which the combustionchamber (110) is annular, the primary combustion zone (52) is annular,the annular primary combustion zone (52) is defined by a first annularwall (148), a second annular wall (146) arranged radially inwardly ofthe first annular wall (148), and the upstream end wall (46), the firstand second annular walls (148,146) are secured at their upstream ends tothe upstream end wall (46), the upstream end wall (46) has a pluralityof apertures, a plurality of secondary fuel and air mixing ducts (88)are arranged around the first annular wall (148) of the primarycombustion zone (52).
 6. A combustion chamber as claimed in claim 2 inwhich the combustion chamber (110) is annular, the primary combustionzone (52) is annular, the annular primary combustion zone (52) isdefined by a first annular wall (48), a second annular wall (146)arranged radially inwardly of the first annular wall (48), and theupstream end wall (46), the first and second annular walls (48,146) aresecured at their upstream ends to the upstream end wall (46), theupstream end wall (46) has a plurality of apertures, a plurality ofsecondary fuel and air mixing ducts (188) are arranged within the secondannular wall (146) of the primary combustion zone (52).
 7. A combustionchamber as claimed in claim 2 in which said plurality of secondary fueland air mixing ducts (88) are arranged circumferentially in a firstannulus radially outwardly of the primary combustion zone (52), thesecondary fuel and air mixing ducts (88) being defined at their radiallyinner extremity and radially outer extremity by a first pair of annularwalls (90,92) and a plurality of walls (94) extending radially betweenthe first pair of annular walls (90,92), and said plurality of secondaryfuel and air mixing ducts being arranged circumferentially in a secondannulus radially inwardly of the primary combustion zone (52), thesecondary fuel and air mixing ducts (188) being defined at theirradially inner extremity and radially outer extremity by a second pairof annular walls (190,192) and a plurality of walls (194) extendingradially between the second pair of annular walls (190,192).
 8. Acombustion chamber as claimed in any of claims 1 to 7 in which at leastone of the secondary fuel injector means (100) comprises a hollowcylindrical member, the hollow cylindrical member has a plurality ofapertures (102) spaced apart axially along the cylindrical member toinject fuel into the secondary fuel and air mixing duct (88).
 9. Acombustion chamber as claimed in claim 8 in which the hollow cylindricalmember extends axially with respect to the axis of the combustionchamber (44).
 10. A combustion chamber as claimed in claim 9 in whichthe hollow cylindrical member extends radially with respect to the axisof the combustion chamber (44).
 11. A combustion chamber as claimed inclaim 9 in which the apertures (102) in the hollow cylindrical memberare arranged to direct the fuel circumferentially.
 12. A combustionchamber as claimed in claim 2 in which the walls (94) extending radiallybetween the annular walls (90,92) are secured to both the annular walls(90,92).
 13. A combustion chamber as claimed in claim 1 in which thesecondary fuel injector means (100) for at least one of the secondaryfuel and air mixing ducts (88) comprises two secondary fuel injectors.14. A combustion chamber as claimed in claim 13 in which the twosecondary fuel injectors (100) are spaced apart radially relative to theaxis of the combustion chamber (44).
 15. A combustion chamber as claimedin claim 1 in which each secondary fuel injector (100) is arranged tosupply fuel to the upstream end of the associated secondary fuel and airmixing duct (88).
 16. A combustion chamber as claimed in claim 1including means (290,292,294) to define a plurality of tertiary fuel andair mixing ducts (288), each tertiary fuel and air mixing duct (288) isin fluid flow communication at its downstream end with a tertiarycombustion zone (286) in the interior of the combustion chamber (44)downstream of the secondary combustion zone (56), each tertiary fuel andair mixing duct (288) has tertiary air intake means at its upstream endto supply air into the tertiary fuel and air mixing duct (288), eachtertiary fuel and air mixing duct (288) has tertiary fuel injector means(300) arranged to inject fuel into the tertiary fuel and air mixing duct(288).
 17. A combustion chamber as claimed in claim 16 in which thetertiary fuel and air mixing ducts (288) are arranged in an annulusoutside the peripheral wall (48), the tertiary fuel and air mixing ducts(288) are defined by a radially inner annular wall (292), a radiallyouter annular wall (290) and a plurality of walls (294) extendingradially between the pair of annular walls (290,292), the radiallyextending walls (294) are secured to at least one of the pair of annularwalls (290,292), each tertiary fuel injector means (300) is locateddownstream of the tertiary air intake means of the associated tertiaryfuel and air mixing duct (288), each tertiary fuel and air mixing duct(288) has an outlet at its downstream end for discharging the fuel andair mixture into the tertiary combustion zone (290), the outlets of thetertiary fuel and air mixing ducts (288) have substantially equal flowareas to produce substantially the same air flow rate through each ofthe tertiary fuel and air mixing ducts (288), the tertiary fuel injectormeans (300) of each tertiary fuel and air mixing duct (288) is arrangedto supply substantially the same flow rate of fuel so that the fuel toair ratio of the mixture leaving each of the tertiary fuel and airmixing ducts (288) is substantially the same.
 18. A combustion chamberas claimed in claim 17 in which the tertiary fuel and air mixing ducts(288) are arranged around the combustion chamber (210).
 19. A combustionchamber as claimed in claim 17 in which the combustion chamber (210) istubular, the peripheral wall (48) of the primary combustion zone (52) isannular and the upstream end wall (46) has a single aperture, theplurality of tertiary fuel and air mixing ducts (288) are arrangedcircumferentially in an annulus radially outwardly of the secondarycombustion zone (56).
 20. A combustion chamber as claimed in any ofclaims 16 to 19 in which at least one of the tertiary fuel injectormeans (300) comprises a hollow cylindrical member, the hollowcylindrical member has a plurality of apertures (302) spaced apartaxially along the cylindrical member to inject fuel into the tertiaryfuel and air mixing duct (288).
 21. A combustion chamber as claimed inclaim 20 in which the hollow cylindrical member extends axially withrespect to the axis of the combustion chamber (210).
 22. A combustionchamber as claimed in claim 20 in which the hollow cylindrical memberextends radially with respect to the axis of the combustion chamber(210).
 23. A combustion chamber as claimed in claim 21 in which theapertures (302) in the hollow cylindrical member are arranged to directthe fuel circumferentially.
 24. A combustion chamber as claimed claim 16in which the tertiary fuel injector means (300) for at least one of thetertiary fuel and air mixing ducts (288) comprises two tertiary fuelinjectors.
 25. A combustion chamber as claimed in claim 24 in which thetwo tertiary fuel injectors (300) are spaced apart radially relative tothe axis of the combustion chamber (210).
 26. A combustion chamber asclaimed in claim 17 in which the radially extending walls (294) aresecured to both the annular walls (290,292).
 27. A gas turbine enginecombustion chamber (210) comprising a primary combustion zone (52)defined by at least one peripheral wall (48) and an upstream end wall(46) connected to the upstream end of the at least one peripheral wall(48), the upstream end wall (46) has at least one aperture (78), primaryair intake means (80,82) and primary fuel injector means (84,86) tosupply air and fuel respectively through the at least one aperture (78)into the primary combustion zone (52), a secondary combustion zone (56)defined by a downstream portion of the at least one peripheral wall(48), the secondary combustion zone (56) is in the interior of thecombustion chamber (210) downstream of the primary combustion zone (52),secondary air intake means (98) and secondary fuel injector means (100)to supply air and fuel respectively into the secondary combustion zone(56), means to define a plurality of tertiary fuel and air mixing ducts(288), each tertiary fuel and air mixing duct (288) is in fluid flowcommunication at its downstream end with a tertiary combustion zone(286) in the interior of the combustion chamber downstream of thesecondary combustion zone (56), each tertiary fuel and air mixing duct(288) has tertiary air intake means at its upstream end to supply airinto the tertiary fuel and air mixing duct (288), each tertiary fuel andair mixing duct (288) has tertiary fuel injector means (300) arranged tosupply fuel into the tertiary fuel and air mixing duct (288), eachtertiary fuel injector means (300) is located downstream of the tertiaryair intake means of the associated tertiary fuel and air mixing duct(288), characterised in that each tertiary fuel and air mixing duct(288) has an outlet at its downstream end for discharging the fuel andair mixture into the tertiary combustion zone (290), the outlets of thetertiary fuel and air mixing ducts (288) have substantially equal flowareas to produce substantially the same air flow rate through each ofthe tertiary fuel and air mixing ducts (288), the tertiary fuel injectormeans (300) of each tertiary fuel and air mixing duct (288) is arrangedto supply substantially the same flow rate of fuel so that the fuel toair ratio of the mixture leaving each of the tertiary fuel and airmixing ducts (288) is substantially the same.
 28. A combustion chamberas claimed in claim 27 in which the tertiary fuel and air mixing ducts(288) are arranged around the combustion chamber (210).
 29. A combustionchamber as claimed in claim 27 or claim 28 in which the tertiary fueland air mixing ducts (288) are arranged in an annulus outside theperipheral wall (48), the tertiary fuel and air mixing ducts (288) aredefined by a radially inner annular wall (292), a radially outer annularwall (290) and a plurality of walls (294) extending radially between thepair of annular walls (290,292), the radially extending walls (294) aresecured to at least one of the pair of annular walls (290,292).
 30. Agas turbine engine combustion chamber (44) comprising a primarycombustion zone (52) defined by at least one peripheral wall (48) and anupstream end wall (46) connected to the upstream end of the at least oneperipheral wall (48), the upstream end wall (46) has at least oneaperture (78), primary air intake means (80,82) and primary fuelinjector means (84,86) to supply air and fuel respectively through theat least one aperture (78) into the primary combustion zone (52), asecondary combustion zone (56) in the interior of the combustion chamber(44) downstream of the primary combustion zone (52), means (90,92,94) todefine a plurality of secondary fuel and air mixing ducts (88), eachsecondary fuel and air mixing duct (88) has secondary air intake means(98) at its upstream end (96) to supply air into the secondary fuel andair mixing duct (88), each secondary fuel and air mixing duct (88) hassecondary fuel injector means (100) arranged to supply fuel into thesecondary fuel and air mixing duct (88), each secondary fuel injectormeans (100) is located downstream of the secondary air intake means (98)of the associated secondary fuel and air mixing duct (88), eachsecondary fuel and air mixing duct (88) has an outlet (104) at itsdownstream end for discharging the fuel and air mixture into thesecondary combustion zone (56), characterised in that the areas of theoutlets (104) of the secondary fuel and air mixing ducts (88) and theflow rate of fuel injected from the secondary fuel injector means (100)are selected such that the fuel to air ratio of the mixture leaving eachof the secondary fuel and air mixing ducts (88) is substantially thesame.
 31. A combustion chamber as claimed in claim 30 in which theoutlets (104) of the secondary fuel and air mixing ducts (88) havesubstantially equal flow areas to produce substantially the same airflow rate through each of the secondary fuel and air mixing ducts (88),the secondary fuel injector means (100) of each secondary fuel and airmixing duct (88) is arranged to suppply substantially the same flow rateof fuel.